1. Field of the Invention
The present invention pertains the transmission of streams of packets through a communications system, and particularly to measuring packet delay in a node which utilizes packet switching technology.
2. Related Art and Other Considerations
Telecommunications generally encompasses two basic types of switching technologies: circuit switching and packet switching. In circuit switching, network resources are continuously reserved for a connection, even when not transferring data. In packet switching, on the other hand, the network resources are reserved for the connection only when a party desires to transmit or the network is transmitting to the party.
In general, packet switching technology is more efficient than circuit switching technology. On the average, with packet switching technology a large number of connections can be transported over a given physical channel. One part of this transport efficiency is accomplished by dividing the traffic into different priority classes. Traffic sent with high priority is sent before traffic with low priority. Further, traffic is only sent through a packet network when there is some information to convey. Hence, give a large number of connections, it can be shown statistically with packet switching technology that more connections can be supported.
In a node that interfaces packet switched networks it is advantageous to also utilize packet switching methods internally, e.g., in the node itself. Such a node is often characterized by an arbitrary number of processing units interconnected by a packet or cell switched back plane. In general, a packet that enters the node is processed by a number of these processing units and sent between them via the back plane or switch factory.
Typically a packet switched node is operated by service provider as part of a network for the benefit of subscribers and customers. A subscriber or customer might complain if the subscriber perceives the level or quality of service to below what has been agreed (e.g., in contracts) between the service provider and the subscriber. For this and other reasons, degraded performance (e.g., delays) in the node or failures at the node need to be monitored on a node basis, and sometimes even at the processing unit level. For example, the service provider must be able to investigate in order either to verify or otherwise address subscriber complaints or other alleged/perceived problems. The service provider might also be proactive and continuously verify that the traffic model is in compliance with the contracts. In either case, mechanisms are needed to measure the actual performance.
Various techniques have already been developed to determine delay between endpoints of a connection. In this regard, for example, current Asynchronous Transfer Mode (ATM) standards provide for the measurement of cell delays and cell loss using certain operations and maintenance (OAM) cells. OAM cells are routed by the ATM network in the same manner as user cells, but are distinguishable from user cells by special values in their headers. Such OAM cells are inserted into a cell stream of user traffic. In order to measure cell delay, the OAM performance monitoring cell contains an optional timestamp field that is used to indicate the time that the OAM cell is inserted into the stream. That timestamp can be compared with the arrival time of the cell at the virtual connection endpoint, and the difference inferred to be the end-to-end delay experienced by a cell along the virtual connection. As an alternative, an OAM loopback cell can be used to measure rout-trip cell delay between an originating endpoint and a destination endpoint of a connection.
Whereas a major thrust of delay monitoring has been between connection endpoints, delay has also been monitored with respect to an intermediate note of a network. Typically such monitoring has taken the form of inserting per node delay reporting information in a special OAM-type cell, or piggybacking on an existing OAM-type cell. For example, U.S. Pat. No. 5,793,976 to Chem et al. uses a management packet having a payload that can be modified by each node along a virtual connection. The payload of the management packet has one or more delay stamp fields which can be utilized either to report a compilation of delay times throughout the various nodes or to keep track of the delay time for each of plural nodes traversed by the management packet. In similar manner, EP 0993146A2 has a directed load/performance (DLP) message from which a delay seen by each node in the network can be obtained by subtracting timestamps which are inserted in the message by each node as the node processes the message.
What is needed, and an object of the present invention, is a technique for monitoring, at a node, the processing or transport delay in the node itself, particularly when the node is based on a packet backplane or packet switch, or software backplane.